The present invention relates to a method and apparatus for controlling an arrangement for delivering a pressure medium, in particular a pump, of a braking system.
German Published Patent Application No. 195 48 248 describes a method and an apparatus for controlling a pump of an electrohydraulic braking system. In this, the hydraulic fluid is injected from a pressure accumulator through valves into the wheel brake cylinder, the pressure accumulator being charged by a pump. In order to make charging of the pressure accumulator as noiseless as possible, the pump is activated with a demand-based pulse/interpulse ratio of a PWM signal, depending on the required volume of hydraulic fluid and a pressure in the pressure accumulator. The PWM signal is modified, as a function of a definable pressure value, with the deviation of the actual pressure from that value (pressure accumulator hysteresis). In this context, a plurality of fixed pressure threshold values are used for activation at appropriate delivery capacity. This Unexamined Application specifically describes the activation of a pump in an electrohydraulic braking system. A braking system comparable thereto is also described in SAE Paper 960991.
Also known are methods and apparatuses in which the return flow pumps in hydraulic braking systems are activated as a function of the generator voltage generated by the pump motor at zero load. Comparable hydraulic braking systems are known, for example, from the offprint of ATZ 95 (1993) Vol. 11, or from German Published Patent Application No. 195 46 682. German Published Patent Application No. 42 32 130 describes, for this purpose, a method and an apparatus for controlling an electric motor-driven hydraulic pump that generates auxiliary pressure for a braking system having an antilock braking control system and/or a drive slip control system. For that purpose, it is equipped with a variable activation clock cycle made up of a pulse/interpulse sequence. The voltage induced in generator fashion by the pump motor during the interpulses (referred to simply as the xe2x80x9cgenerator voltagexe2x80x9d) is analyzed as an indication of the pump rotation speed. By differentiating this generator voltage (as the rotation speed actual value) with a reference variable for the pump motor rotation speed created in an antilock braking or drive slip control system, a difference variable is then made available to a downstream controller. The output signal of the controller is used to create the pulse-width modulated adjusting signal for pump activation. The drive motor of the hydraulic pump is switched on and off in time with this pulse-width modulated adjusting signal.
It has been found that the known method and the corresponding apparatuses are not capable of yielding optimum results in every respect. It is found, for example, at low temperature or in the presence of other disturbances, that the delivery capacity of the pump in clock-timed operation, regulated by the generator voltage, is not sufficient. In addition, there is no guarantee with the known methods that a desired pressure level will be reached within a time that is definable as a function of the situation. This applies equally to pressure rise and pressure drop. For example, the need to deliver the pressure medium more quickly or more slowly out of an accumulator in the braking system in order to take into account a desired pressure drop, not only in the wheel brake cylinders but also in the rest of the braking system, is insufficiently met. Similarly, pressure medium is not conveyed into the entire braking system and/or into a pressure accumulator with an exact time stipulation, i.e. in terms of achieving a specific pressure level within a particular time period.
The activation signal of the arrangement for delivering the pressure medium, and/or its creation, especially in the case of a PWM signal, is dependent on at least one condition. One condition for creating an activation signal for activating the arrangement for delivering the pressure medium, in particular the pump, of a braking system, is constituted, according to the present invention, at least as a function of a pressure gradient variable.
This pressure gradient variable represents an actual value of a pressure gradient of the pressure medium in the braking system. The condition for creation of the activation signal is referred to as the xe2x80x9cdependency mode,xe2x80x9d and is configured, for example, as a comparison of the pressure gradient variable to various pressure gradient threshold values, and/or as an analysis of a characteristics diagram with the pressure gradient variable as one characteristics diagram variable, and/or as a stipulation of the pressure gradient variable as a command variable in a control loop.
In addition, as a further dependency mode, it is also possible to ascertain on the basis of a definable activation signal having a definable switched-on duration within an activation clock cycle, by simple further calculationxe2x80x94for example by addition or subtraction of time steps to or from the switched-on duration, by multiplication or division, or by other operationsxe2x80x94an adapted activation signal based on the original one and following therefrom, from which a new pressure gradient variable then results.
The condition used hitherto for activating a pump of a braking system (the dependency of the activation signal on the generator voltage) can now also be advantageously replaced and/or supplemented, in situations in which the previous method does not always yield optimum results, by a further condition: the dependency of the activation signal on a pressure gradient.
The method according to the present invention thus has the advantage that in its context, the pulse/interpulse ratio of the activation signal or the switched-on time of the pump is modified within one activation clock cycle, for example, in such a way that the pressure in the accumulator at least reaches a predefinable pressure change. This applies equally to a pressure rise and a pressure drop. This ensures that a desired pressure level is also achieved within a predefinable time. On the one hand it is thereby possible to achieve a minimum pressure rise or a minimum pressure drop in the pressure medium as well as a maximum pressure rise or maximum pressure drop, i.e. a maximum pressure gradient; and on the other hand, an exact time stipulation for reaching a specific pressure is ensured.
In addition, the delivery capacity and the rate of the pressure rise or pressure drop can be set by using different pressure gradient values used as threshold values, and, associated with them, predefinable switched-on durations within an activation clock cycle.